The present invention relates to aircraft engines and, more particularly, to a pulse detection circuit for use with any gas turbine jet engine utilizing an AC signal with a one-per-revolution index for indication. Specifically, this invention is applicable to aid in balancing the fan module of a gas turbine engine.
The General Electric CF34-3A1 regional jet engine uses a bearing lock nut with twenty-eight teeth to generate a magnetic field and is detected using a magnetic pick-up speed sensor. The sensor signal is proportional to fan speed and outputs twenty-eight pulses for every fan rotor revolution. One of the twenty-eight pulses is a lower amplitude because of a shaved tooth on the bearing lock nut that causes a larger gap between the tooth and the speed sensor.
An output signal indicative of the low pulse for the once-per-revolution is needed to balance the engine fan module. Unfortunately, there is a lack of test instrumentation capable of discriminating all of the high pulses and outputting only the low pulse for once-per-revolution indication. This lack of available instrumentation results in a lengthy procedure to balance the fan. This includes mounting temporary instrumentation to generate a one-per-revolution signal. Presently, special tape and an optical sensor are added to the engine fan during manufacturing. The system has to be taken off the engine when the procedure is complete. This takes over twelve man hours and consumes even more time when an engine is installed in an aircraft because the fan is not easily accessible.
Limitations of other equipment used to detect pulses is that they can not work over a wide range of amplitude and frequency without adjustment. This is because the amplitude of a speed sensor signal varies with frequency, speed sensor gain, and the gap between the sensor and the material generating the magnetic field, i.e., the bearing lock nut. This makes it difficult to have a fixed threshold to detect an index with a varying amplitude and frequency.
It is therefore highly desirable and an object of the present invention to provide a system for detecting an index signal without removing the engine or adding temporary instrumentation.
Another object of the present invention is to provide a system which allows for detection of the index signal that is part of the fan speed signal.
Another object of the present invention is to provide such a system for use with speed sensor generated signals.
These objects and other features and advantages will become more readily apparent in the following description when taken in conjunction with the appended drawings.